Understand the Delphi Class (vii)

What is polymorphism? My understanding is that the same method will have different implementations in different objects, that's all.

unit Unit1;
interface
uses
　Windows, Messages, SysUtils, Variants, Classes, Graphics, Controls, Forms,
　Dialogs, StdCtrls;
type
　TForm1 = class(TForm)
　　Button1: TButton;
　　Button2: TButton;
　　Button3: TButton;
　　procedure Button1Click(Sender: TObject);
　　procedure Button2Click(Sender: TObject);
　　procedure Button3Click(Sender: TObject);
　end;Three classes are defined, and the second two classes inherit from the first class Tbass　TBass = class
　　procedure alert; virtual;

{With the virtual designator, this method may be modified (or overwritten)}

{This method is called a virtual method}　end;
　TChild1 = class(TBass)
　　procedure alert; override; {override 表示修改父类的同名方法}
　end;
　TChild2 = class(TBass)
　　procedure alert; override;
　end;
var
　Form1: TForm1;
implementation
{$R *.dfm}
{ TBass }
procedure TBass.alert;
begin
　ShowMessage('is Bass');　　{同一个方法, 在不同类里有不同的实现}
end;
{ TChild1 }
procedure TChild1.alert;
begin
　ShowMessage('is TChild1'); {同一个方法, 在不同类里有不同的实现}
end;
{ TChild2 }
procedure TChild2.alert;
begin
　ShowMessage('is TChild2'); {同一个方法, 在不同类里有不同的实现}
end;Test 1:procedure TForm1.Button1Click(Sender: TObject);
var
　bass: TBass; {定义 TBass 的变量}
begin
　bass := TBass.Create;
　bass.alert; {is Bass}
　bass.Free;
end;Test 2:procedure TForm1.Button2Click(Sender: TObject);
var
　bass: TBass;　　　　　　{注意还是定义 TBass 的变量}
begin
　bass := TChild1.Create; {但这里是通过 TChild1 建立对象}
　bass.alert; {is TChild1}
　bass.Free;
end;Test 3:procedure TForm1.Button3Click(Sender: TObject);
var
　bass: TBass;　　　　　　{注意还是定义 TBass 的变量}
begin
　bass := TChild2.Create; {但这里是通过 TChild2 建立对象}
　bass.alert; {is TChild2}
　bass.Free;
end;{Summary: Now the same is the Bass.alert method, through the implementation of different objects, with different functions, this is polymorphism!}

End.